#include <iostream>
#include <iomanip> 
#include <cmath>

using namespace std;

int main() {
	double a, b, c;
	cin >> a >> b >> c;
	double p = (a + b + c) / 2;
	double area = sqrt(p * (p - a) * (p - b) * (p - c));
	cout<< fixed << setprecision(2) << area <<endl;
    return 0;
}
